Trypanosoma cruzi is the agent of Chagas' disease in the Americas and T. brucei spp. is the cause of sleeping sickness in humans and nagana in animals. Trypanosomes, as well as trypanosomatids like Leishmania spp., have to tightly regulate gene expression to adapt to different environments in vertebrate and insect hosts. However, control of gene expression at transcription initiation, the most frequent and highly used mechanism from bacteria to mammalian cells, is likely to be not relevant in Trypanosomes. Thus, post-transcriptional mechanisms seem essential for the development of parasite stages and survival in the hosts. As a consequence, trypanosomes constitute an interesting model cell to study post-transcriptional events leading to transcript translation/degradation that could be used to develop alternatives for disease control. Results from several groups, including our group, suggest the importance of the modulation of mRNA half-life in gene expression. This process involves motives in the transcript and RNA-binding proteins, both forming part of ribonucleoprotein complexes which can be detected "in vivo'. The hypothesis is that the different composition of these complexes renders the transcript prone for destabilization/degradation or translation into a protein. The long-term objectives of the work proposed are the characterization of mRNAs and RNA-binding proteins in different ribonucleoprotein complexes during the mRNA pathway and how their composition results in transcript degradation/translation during parasite development. For this purpose, the mRNAs present in specific ribonucleoprotein complexes bearing previously characterized RNA-binding proteins will be identified. Then, protein partners of known RNA-binding proteins present in ribonucleoprotein complexes "in vivo" will be characterized. Finally, the biological relevance of ribonucleoprotein complexes during parasite development will be studied. Parasites knocked-out/over-expressing a RNA-binding protein will be constructed to analyze the fate of transcripts. The information obtained will contribute to the understanding of a relevant process for trypanosome survival and to the identification of potential targets for unconventional drug development.